Inductively coupled plasma (ICP) process reactors generally form plasmas by inducing current in a process gas disposed within the process chamber via one or more inductive coils disposed outside of the process chamber. The inductive coils may be disposed externally and separated electrically from the chamber by, for example, a dielectric lid. When radio frequency (RF) current is fed to the inductive coils via an RF feed structure from an RF power source, an inductively coupled plasma can be formed inside the chamber from an electric field generated by the inductive coils.
AC/DC driven heaters are widely used to control surface temperature of the ceramic window isolating the process chamber from the ICP source, mainly to control the overall deposition on the dielectric window facing the plasma in the process chamber. In order to eliminate RF coupling of the ICP source to the AC/DC powered heated elements, the heaters are embedded within a aluminum or any conductor shield to prevent any interference of the RF to the AC/DC heaters. The aluminum shield also acts as partial RF shield between the ICP power coupled to the vacuum chamber and controls the overall sputter rate at the window due to voltages developed at the shield assuming the shield is floating (i.e., the shield is not grounded). However, power coupling to some conventional shield reduces the overall power coupled to the vacuum chamber in situations when the shield consists of a closed loop that current might be induced in. In particular when dual mode ICP coils are used, the coupling in the out-of-phase case may be significant due to the intensity of the magnetic field, leading to higher power losses, and lower power coupling efficiency compared to the in-phase case. The out-of-phase coupling is mainly used to eliminate or reduce M-shape signatures leading to non-uniformities.
Accordingly, the inventors have devised a substrate processing apparatus to better control RF coupling and substrate processing uniformity at the substrate surface level.